Merge branch '25-prediction-refactoring' into 'master'

#25 use RailEnvActions enum instead of hardcoded action ints

Closes #25

See merge request flatland/flatland!51

flatland/envs/predictions.py

@@ -5,6 +5,7 @@ Collection of environment-specific PredictionBuilder.
 import numpy as np
 
 from flatland.core.env_prediction_builder import PredictionBuilder
+from flatland.envs.rail_env import RailEnvActions
 
 
 class DummyPredictorForRailEnv(PredictionBuilder):
@@ -41,12 +42,7 @@ class DummyPredictorForRailEnv(PredictionBuilder):
         prediction_dict = {}
 
         for agent in agents:
-
-            # 0: do nothing
-            # 1: turn left and move to the next cell
-            # 2: move to the next cell in front of the agent
-            # 3: turn right and move to the next cell
-            action_priorities = [2, 1, 3]
+            action_priorities = [RailEnvActions.MOVE_FORWARD, RailEnvActions.MOVE_LEFT, RailEnvActions.MOVE_RIGHT]
             _agent_initial_position = agent.position
             _agent_initial_direction = agent.direction
             prediction = np.zeros(shape=(self.max_depth, 5))